1. Field of the Invention
The present invention relates generally to part holding devices for use with coordinate measurement machines and, more particularly, to a universal part holding device and method for constructing such a holding device for effectuating three dimensional measurement of such a part and which is easily adaptable to any portable or stationary three dimensional measurement system.
2. Description of the Prior Art
Coordinate measurement machines and part holding fixtures are well known in the art. Such machines are usually considered to be very precise in the measuring of tooling and miscellaneous workpieces and in certifying tooling. The coordinate measurement machine (or CMM) is typically assembled in a separate climate controlled room to achieve the required accuracy during part measurement. Examples of measurement systems include portable coordinate measurement machines, laser trackers, photogrammetry systems and the like.
A particularly useful measurement device is the camera-based and electro-optical portable coordinate measurement machine providing fast and accurate three-dimensional data and being commercially known in one variant as the Metronor system. Typically, the Metronor system includes two such camera devices arrayed in a spaced and desired angular orientation relative to the part holding fixture and cooperating with a portable light pen having one or more active LED's (light emitting diodes). The light pen is capable of being placed against selected points upon the part held within the fixture so that the cameras can register extremely accurate measured distances to provide detailed three dimensional data.
Along with a specified means for measuring part dimension, a CMM holding fixture is designed and built for each part or sub-assembly. The holding fixture consists of pins, net blocks, and clamps to hold parts at specified orientations. Each pin and net block is located within a specified build tolerance. Known problems associated with conventional CMM's include their tendency to lose accuracy over time and to require time expensive recalibration to retain their accuracy.
An example of the prior art is illustrated by U.S. Pat. No. 4,848,005, issued to Ercole et al., which teaches a rearrangeable supporting and positionable fixture for parts measurable on a gauging machine. The fixture includes a number of rearrangeable elements, a hand operated by a robot and designed to position the elements on a reference table. Similarly, U.S. Pat. No. 5,6245,959, also issued to Ercole, again teaches a fixture for positioning and supporting parts for measurement. According to the '959 disclosure, the reconfigurable supporting elements include first portions positionable on the reference surface of the machine. Second portions positionable in relation to the first portions are arrayed in a direction perpendicular to the reference surface. A reference tool positioned automatically by the machine and presenting a three-dimensional reference for the second portion of the supporting elements, the supporting elements being moved manually over the reference surface and so that the second portion contacts the reference tool prior to being clamped in position. U.S. Pat. No. 5,848,480, issued to Sola et al., teaches a still further variation of a reconfigurable supporting fixture for a measuring machine and which again includes a number of reconfigurable supporting elements.
The reference systems as discussed above are based upon the calibration of the measurement machine. The positioning of all components that establish the reference system are held to machine-specific build tolerances and require a very high degree of precision. Accordingly, it is essentially impossible to create an error-free reference system for part measurement as a result of these build tolerances. The reconfigurable fixtures are further specifically designed for a single type of measuring device and further possess limited load capacity and limited capability for moving supporting elements. Furthermore, the holding fixtures described above only support the parts in a vertical manner and no provision is made for additionally securing the part to be measured in a horizontally and rotatably restricted fashion.
Referring to U.S. Pat. No. 5,107,599, issued to Marincic et al., a universal part holding fixture is disclosed for a coordinate measurement machine which utilizes a plurality of ball detent retainers bolted to a base plate to allow part holders to be set up and removed with improved speed and accuracy. Pluralities of threaded and non-threaded holes are provided formed through the base and receive clamping portions of associated part holders. Each four hole set must be machined in the base at known precise locations. The base must be precisely aligned and squared to the CMM's reference system. At one corner of the bases a sphere is inserted into a mounted retainer and measured to find the center point. The CMM translates the reference system to this center point. Accordingly, the device of Marincic creates two sources of potential error with this application, one for the build tolerances of the CMM and one for the positional error of the base. As with the prior described references, each part holder in Marincic is uniquely tailored to each part to be supported.
U.S. Pat. No. 5,909,939, issued to Fugmann, discloses a CMM machine having a stationary base plate, a workpiece receptacle, a tracing head movable relative to the baseplate, and a supply, control and evaluation device. The baseplate includes legs which are mounted pivotably on at least three fixed locations on the baseplate and on a body which carries the tracing head. The length or the inclinations of the legs can be adjusted in an accurately measurable manner.
Finally, U.S. Pat. No. 5,829,151, issued to Collier et al., teaches a multi-axis part positioning system for holding a component relative to an established reference frame and which is usable with any type of coordinate measurement or numerical control machine. The fixture includes a base fixable relative to the established reference frame, and a plurality of stanchions continuously movable along a plane defined by a surface of the base. Each of the stanchions are engaged by a dual-axis linear stepped motor that actuates the associated stanchions along a dual axis (x,y) grid pattern machined into the base. The stepper motor is also coupled to a vertical jack screw so that a height of the screw is established by a determined number of rotations of the stepped motor. As with the afore-described references, the fabrication of the part positioning system according to Collier is fairly costly and requires a very high degree of precision.